1. Field of the Invention
This invention relates to an aspherical single lens homogenizer which converts a laser beam emitted from a laser into a uniform power density beam. High power lasers, for example, CO2 lasers or YAG lasers, are utilized for shearing, welding or annealing of metal. Beams emitted from lasers are monochromatic, parallel and coherent but non-uniform in spatial distribution of power. Power density is higher at the middle of the beam and lower at the periphery.
This application claims, the priority of Japanese Patent Application No. 2003-371368 filed on Oct. 31, 2003, which is incorporated herein by reference.
In an ideal case, spatial power density of an inherent beam emitted from a laser takes a Gaussian distribution. The beam having the Gaussian power distribution is named a Gaussian distribution beam or, in short, a Gaussian beam. Conventional laser processing apparatus shear, weld or anneal by converging an inherent Gaussian laser beam by a lens and shooting the converged Gaussian beam to a metal object.
Some sorts of utilities which make use of power of laser beams require uniform-power density beams within a spatial scope. The uniform-power density beam signifies a step function type beam which has a constant power density in the scope and has zero power out of the scope unlike ordinary Gaussian beams. The step-functioned, uniform-power beam is simply called a “tophat”, because the distribution function resembles a tophat. A section of the tophat is a circle, square, rectangle, ellipsoid and so on.
There are a variety of means for converting a Gaussian beam into a tophat beam. A lattice-divided complex mirror makes a tophat beam by reflecting a Gaussian beam by lattice-divided micro-mirrors and superposing all the reflected rays onto a single tiny area. The power density within the tiny area is essentially constant, since the input Gaussian beam is divided into a plenty of small parts and all the parts are superposed on the narrow area. The lattice mirror method is suitable for making a uniform rectangle on an image. But, the phases are disordered at random. It takes much time and cost to produce the lattice-divided mirror.
Another contrivance for producing a tophat beam is a diffractive optical element (DOE), which is a glass plate having M×M pixels with different step heights. But cutting of many different step height pixels requires long time and high cost.
Someone contrived aspherical lenses for reforming a Gauss beam emitted from a laser into a tophat beam having a uniform power distribution. The optical device of reshaping a Gaussian beam into a tophat beam is called a “homogenizer”.
A lens-type homogenizer uses two aspherical lenses. One is an aspherical lens reshaping a Gaussian power distribution into a uniform power distribution. This lens is called an “intensity-conversion” lens. Although the power distribution is reformed into a uniform density distribution, phases of waves are randomly disordered by the intensity-conversion lens. The other is an aspherical lens for remedying random phases into a regular phase having the same value on a plane vertical to the beam. The lens is called a “phase-compensation” lens.
Then a conventional lens-type homogenizer requires at least two lenses. Conventionally, homogenizer=intensity-conversion lens+phase-compensation lens. The two lens homogenizer is called a “binary lens homogenizer”. The homogenizer is an optical system for converting a Gauss beam of a radius “a” into a tophat beam of a radius “b”. The final beam should be a parallel uniform power (tophat) beam of a radius b. Why parallelism is required for the final beam is that the parallelism enables the propagating tophat beam to maintain a regular phase relation and uniform power at any positions on a propagation path. In addition, a parallel beam is suitable for enlarging, diminishing or reshaping.
The intensity-conversion lens, which converts a Gaussian beam into a uniform-power beam, cannot be an ordinary spherical lens. The intensity-conversion lens should be an aspherical lens. The phase-revising lens should be also an aspherical lens. The conventional binary lens homogenizer has perfect freedom of giving the initial beam an arbitrary diameter 2a and giving the on-image beam an arbitrary diameter 2b. The binary lens homogenizer can realize all three relations of a<b, a=b, or a>b.
Not a binary lens homogenizer but a single lens homogenizer is an object of the present invention. The single lens homogenizer would have an advantage of reducing the cost by half. The present invention aims at a single lens homogenizer which converts a large Gaussian beam (2a beam diameter) into a narrow uniform power beam (2b beam diameter) (a >b). The uniform-power beam is a parallel beam no more on the image. Lack of freedom of the one lens deprives the beam at the image of parallelism and coherency. The one lens homogenizer of the present invention deviates from the inherent definition of a homogenizer which converts a Gaussian input beam into a uniform-power coherent, parallel beam. The on-image beam is neither parallel nor coherent. The present invention gives such a single lens modified homogenizer.
A subject matter is a tilt. The tilt signifies that a lens inclines a bit to a normal plane which is defined to be vertical to an axial beam line. A position of the center is correct and the axial line pierces the center of the lens. The tilt is such an error that a lens plane is not exactly vertical to the optical axis. If a lens is tilted, output beams cross the optical line at points deviating from normal positions. The error is called “coma” aberration. The purpose of the present invention is to give a single lens homogenizer which can suppress the tilt error.
2. Description of Related Art
U.S. Pat. No. 3,476,463 (Justin. L. Kreuzer) proposed a binary lens homogenizer having an intensity-converting lens of converting a Gaussian beam into a tophat beam and a phase-compensation lens of recovering an in-phase beam and revising slanting rays into parallel rays. Namely, the phase-compensation lens has functions of restoring phase, coherency and parallelism. The beam emitted from a laser is called an “input beam”. The beam having passed the intensity-converting lens is called an “intermediate beam”. The beam having passed the phase-compensation lens is called an “output beam”. U.S. Pat. No. 3,476,463 converts an input parallel Gaussian beam with a radius R into an output parallel, tophat beam with a radius R.
U.S. Pat. No. 3,476,463, which makes the parallel 2Rφ tophat beam from the parallel 2Rφ Gaussian beam, makes use of two lenses with the same diameter. The input laser beam is coherent (in-phase) and parallel. U.S. Pat. No. 3,476,463 required parallelism, coherency and in-phase property to the output beam. The intensity-conversion lens breaks the parallelism, coherency and in-phase property once. The phase-compensation is indispensable for recovering the parallelism, coherency and in-phase property.
What makes uniform power at the sacrifice of a regular phase and direction is the intensity-converting lens. What restores phase, coherency and parallelism is the phase-compensation lens.
The pre-posed intensity-converting lens has a flat front surface and a concave rear surface. The concave rear surface is aspherical. Namely, the intensity-converting lens is a flat/concave lens. Since the tophat beam (2b) is larger than the Gaussian beam (2a) (a<b), the central portion of the rear surface of the intensity-converting lens is concave for expanding the beam.
The post-posed phase-compensation lens has a convex front surface for revising the expanding beam rays into parallel rays. The rear surface of the phase-compensation lens is a flat surface for outputting parallel rays without further refraction. Then the phase-compensation lens is a convex/flat lens in U.S. Pat. No. 3,476,463.
The intensity-converting lens is a flat/concave lens. The phase-compensation lens is a convex/flat lens. The front of the lens system and the rear of it are flat as a whole. Inner facing surfaces are concave and convex.
It is unavoidable that the outer both surfaces are flat surfaces in the binary lens homogenizer of U.S. Pat. No. 3,476,463. The laser beam is an in-phase, plane wave with a single wavelength. Monochromaticity enables the laser beam to maintain in-phase property in propagation. The output beam should be also an in-phase, monochromatic, plane wave. Thus the rear surface of the phase-compensation lens should be flat. The binary lens homogenizer is composed of a set of flat/concave+convex/flat lenses.
Indeed, the both flat surfaces enable the binary lens homogenizer to settle simple differential equations, analyze and solve the differential equations exactly and give the concave shape of the intensity-converting lens and the convex shape of the phase-compensation lens in U.S. Pat. No. 3,476,463. Although the differential equations cannot be analytically integrated. A computer can integrate the differential equations and can give exact shapes of the aspherical concave, convex surfaces.
The input beam radius R is maintained to be the output beam radius R (R→R). The magnification rate is 1:1 in U.S. Pat. No. 3,476,463. There are many advantages if the output beam is a large, parallel, in-phase, monochromatic tophat beam.
High-quality of the tophat beam enables mirror optics to repeatedly reflect the large parallel beam to an arbitrary position with maintaining the in-phase property, parallelism, coherency and monochromaticity. The high-quality allows a galvanomirror to sway the parallel beam right and left with maintaining the uniform phase. Many holes can be two-dimensionally bored on an object plate by producing a pulsation Gaussian beam by a pulse laser, converting the Gaussian into a tophat, parallel, coherent beam by the binary lens homogenizer, and oscillating the parallel, coherent beam by the galvanomirror.
Furthermore, the strong, wide, parallel tophat beam enables a diffractive optical element (DOE) to produce many (hundreds to thousands) one-dimensionally or two-dimensionally equivalent beams at equal intervals which bore, weld or anneal many one-dimensionally or two-dimensionally equivalent points at a moment. Parallel, in-phase, coherent beams, which are prepared by the binary lens homogenizer, are advantageous. Then the binary lens homogenizer of U.S. Pat. No. 3,476,463 is useful.
Japanese Patent Publication No.10-153750, “LASER BEAM SHAPING OPTICAL PARTS”, proposed a binary lens homogenizer which makes an enlarging or shrinking beam having a tophat power distribution on an image plane. The homogenizer has an intensity-converting lens and a phase-revision lens. But the rate of sizes of an input Gaussian beam to an output tophat beam is not 1:1 but 1:M or M:1. The final beam going out of the phase-revision lens has uniform-power within a definite length. The output beam is either diverging or converging. The output beam has parallelism, in-phase property no more. Instead of providing the magnifying or shrinking power, the binary lens homogenizer lost the parallelism, in-phase property and coherency.
Consequently, U.S. Pat. No. 3,476,463 proposed a 1:1 homogenizer satisfying theoretically ideal parallelism, coherency, and uniform beam power distribution, but Japanese Patent Publication No. 10-153750 sacrificed the parallelism and coherency for obtaining the possibility of magnifying or shrinking. Lens designing is carried out by wave-optical calculations and several solutions satisfying uniform beam power distribution are calculated.
However, an input beam is not always swayed by a galvanomirror and there is not always necessity to divide the beam into a lot of beams by a DOE.
Moreover, there is also a possibility that the laser power of a light source is weak and insufficient to divide the beam into a plurality of output beams. In this case, one beam is converged on one point of an object as it is. In comparison with the above-mentioned sophisticated galvanomirror or DOE, this case is behind them by some step. But, use for shining one laser beam only at the one point of the object sometimes occurs. This case is of a type of a>>b shown by the above-described representation.
For example, it is assumed that a wide 10 mmφ Gaussian laser beam should be converged into a 100 μmφ beam for microprocessing of welding, cutting or annealing. Use of a simple converging lens makes only a narrow 100 μm Gaussian beam. Gaussian beams are sometimes unsuitable. When a piercing hole is bored on a metal plate by irradiating a narrow Gaussian beam, the bored hole is often conically tapered. When a cavity is formed on an object metal by the Gaussian beam, the cavity sometimes becomes a conical one instead of a perfect cylindrical cavity. Due to the weak power density, walls of the hole or the cavity are apt to incline to an axial line. Exact boring of a perfect cylindrical hole requires an equi-power (tophat or supergaussian) beam.
U.S. Pat. No. 3,476,463, which has the intensity-converting lens and the phase-compensation lens, would be able to make a 10 mmφ tophat beam from a 10 mmφ Gaussian beam. Then the 10 mmφ tophat beam would be converged by a simple converging lens into a 0.11 mmφ tophat beam. But the two-step and two-lens optical system would be undesirable. The converging lens requires a very long path. The shrinking rate is 1/100. The lens/mask distance is denoted by s and the lens/image distance is denoted by t. The 1/100 rate requires s:t=100:1. The converging lens should have an effective diameter of about 20 mmφ, because the input beam has 10 mmφ. The lens, which converges 10 mmφ into 0.1 mmφ, requires a long t. For example, if it is assumed to be t=50 mm, s should have a far long length s=5000 mm=5 m. The big rate of 100:1 and the big laser beam needs such a long optical path before and after the converging lens. The long optical length is undesirable.
The extreme case of 2a>>2b requires a new type homogenizer which can immediately make a narrow equi-energy (tophat or supergaussian) beam unlike U.S. Pat. No. 3,476,463 which maintains beam diameters (R→R: 2a=2b). The outstanding case allows phase disorder, non-parallelism and incoherency, since the homogenized beam is fully consumed on the image plane. In-phase, parallelism, coherency and monochromaticity are important for propagating as a plane wave without disturbing phase and directions.
When a purpose of laser apparatus is heat processing of boring, welding or annealing of an object by irradiating a laser beam thereon, the homogenized beam, which has been entirely absorbed by the object, propagates no more. Thus, the in-phase property, parallelism and coherency are unnecessary for the homogenized beam which is fully consumed just at the moment of being homogenized.
The object beam is far smaller (2a>>2b) than the input laser beam. The beam should be converged by a lens. It is more desirable to shorten the length of optical systems at the sacrifice of the coherency, parallelism and in-phase property. The purpose itself is novel. The new purpose requires a single lens homogenizer. Then the concept of the one lens homogenizer is also novel.
In the case of the small object beam, it is preferable to build a homogenizer by a single lens. The homogenized tophat beam has a diameter 2b quite smaller than the diameter 2a of the Gaussian beam. The homogenized tophat beam is just consumed on the image plane for welding, annealing or shearing objects without further propagation. What is important is only the equi-energy distribution (tophat or supergaussian). The equi-energy beam dispenses with parallelism, in-phase property and coherency.
No single lens homogenizer has existed hitherto. A single lens homogenizer itself is a novel idea itself. The concept of the single lens homogenizer is novel. The single lens homogenizer should have a function of a converging lens and an intensity-converting lens. The novel homogenizer has the function of phase-compensation no more. The lens homogenizer should have positive refractive power for making a uniform-power beam within a small region (r≦b) on the image plane.
FIG. 13 denotes a single lens homogenizer with a front surface S1 and a rear surface S2. S1 and S2 have possibility of being flat, convex or concave. Thus concrete shapes are not shown in FIG. 13. The single lens homogenizer should have the function of converting an input parallel, wide (diameter=2a) Gaussian beam into a converging beam which has narrow (diameter=2b) uniform-power density on the image plane. This is an optical system including the single lens homogenizer at which the present invention aims.
The object is a novel single lens homogenizer. The intensity-conversion lens of Justin's binary lens homogenizer has a flat front surface (S1) and a concave rear surface (S2). Since the incidence laser beam is an in-phase parallel beam, a non-flat incidence surface would induce difficulty on designing a shape of the lens. Then the incidence front surface (S1) was flat in Justin's intensity-conversion lens. The reason why the rear surface was concave is to diverge a central high power part of the Gaussian beam. Most converging lenses have, in general, a convex front surface and a convex rear surface. The shapes of lenses are expressed as flat/convex, convex/flat, convex/concave and so on. Adjectives before “/” mean the shape of the front surface (S1) and the other adjectives after “/” mean the shape of the rear surface (S2).
The novel, virtual single lens homogenizer should have a function combining the flat/concave intensity-conversion lens with the convex/convex converging lens. Since the virtual single lens homogenizer should be a combination of flat/concave and convex/convex, it is assumed to be a flat/convex lens. The single lens homogenizer is novel by itself, since prior lens homogenizers have included at least two lenses. We can show no prior art of a single lens homogenizer.
It is supposed that a virtual lens combining a flat/concave intensity-conversion lens with a convex/convex converging lens should be a flat/convex single lens which has a flat front surface and a convex rear surface. The convergence means not spot convergence but uniform-power (tophat or supergaussian) area convergence of a radius “b”. A spherical convex surface is insufficient. An aspherical convex surface would be required for making a uniform-power definite area beam.
The inventors of the present invention have deliberated, so that use of the one lens homogenizer is novel and the homogenizer itself has novelty. Actually, the inventors have designed a flat/convex lens and have carried out experiments in forming of microspots having uniformed power density, that is, tophat-like power density. The inventors have finally understood that a flat/convex lens used for the one lens homogenizer is very weak on a tilt. The tilt is a lens inclination in relation to an axis of the lens. Even a very small tilt of the lens such as one minute or ten minutes (60 minutes=1 degree) induces large disorder of power density on an image plane. So, a one lens homogenizer influenced little by the tilt of the lens has been desired and the present invention reaches it.
But, phases have relations to energy and there is a possibility that disorder of the phase interferes with strict equipartition of the energy. Actual energy is given by the square of amplitude of a wave function and the energy in the present invention should be given as the square of amplitude of a wave function by calculating the wave function with a wave optical means. However, to solve wave optical equations is extremely difficult. When an optical system is arranged at a correct position, an approximate solution can be obtained, but when an optical system has a tilted lens, the equations are too complex to calculate a difference quantity of the energy with a wave optical means.
The inventors had to use a means of obtaining a route of beams one by one by ray tracing according to geometrical optic technique. So, the phase is not analyzed. The power has relations to the phase because the power is a cosine of phase difference if the phase is not uniformed. The ray tracing on the basis of geometrical optics cannot strictly analyze the power of beams. So, in the present invention, a lot of beams assumed at regular intervals having equal energy density in a Gaussian beam are traced, and beams with uniform distribution on an object plane are considered to be of a tophat type.
A new homogenizer of the present invention having one intensity-conversion lens has a shape of a convex lens which reshapes a wide parallel Gaussian beam into a small spot having a radius b on the object plane. The lens shape is a little different from a conventional converging convex lens but is similar to it, since the lens does not converge on one point but forms the beam with uniform distribution having the finite radius b.
When a homogenizer device is actually manufactured, manufacture errors such as lateral disorder between a lens and an axis of the lens, disorder in an axial direction, and inclination of the lens occur. Here, a problem to be solved is the inclination of the lens in relation to an axis of the lens. The inclination of the lens is called a tilt. When a surface of a lens is positioned perpendicularly exactly to an axis of the lens, a tilt error is zero degree. But actually, it is difficult to realize the zero degree of the error. Irregularity always happens. Actually, alignment is carried out and the center of a beam coincides with the center of an object, but peripheral beams straggle widely from the object. In the case of homogenizers, deviation of the beams caused by the tilt is conspicuous and the beams are deviated even when the tilt is one minute (1/60 degree).
A variety of installment errors accompany actual optical apparatuses. It is desirable that the deviation of beams induced by the installment errors should be small. Namely, a wider tolerance (allowance for errors) for the tilt facilitates to install optical parts with more ease. A larger tilt tolerance is more favorable for installing single lens homogenizers in optical apparatuses.